Alternating Current (AC) electrical measurements are used in a wide variety of applications and may be performed for a variety of electrical quantities including, for example, voltage, current, capacitance, impedance, frequency, phase, power, energy, and resistance. These tests and measurements include those relating to designing, evaluating, maintaining and servicing electrical circuits and equipment range from high voltage electrical transmission lines operating at hundreds of kilovolts (kV) and kiloamps (kA) to industrial/medical/residential electrical and lighting, typically 400V/240V/100V and 30/15 A, to a wide variety of industrial/scientific/medical/consumer electrical and electronic devices operating at voltages of hundreds of mV and currents of a few mA.
Within a variety of AC current applications and AC current test equipment systems AC comparator bridges and AC current transformers are employed to provide the required dynamic range, accuracy, and flexibility. AC current bridge configurations remove many of the issues associated with achieving making measurements at accuracies of a part, or few parts per million, such as insensitivity to lead resistances, excellent ratio linearity, excellent ratio stability, and a high level of resolution. AC current transformers, importantly, isolate the measuring instruments from what may be very high voltage in the monitored circuit and when the current in a circuit is too high to be directly applied to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. They also allow accurate high current generation from precision lower current sources and isolation of the precision source from external variations.
Accordingly many sources and measurement systems for alternating current power systems have a current transformer at their output and input stages respectively. Over the past approximately 180 years whilst a wide variety of types of electrical transformer are made for different purposes these, despite their design differences, employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts. Over this period many techniques have been developed to improve the accuracy of the current transformer. Among them, the dual stage current transformer, described in the work of Brooks and Holtz in “The Two-Stage Current Transformer” (AIEE Trans., Vol. 41, pp 382-393, 1922) still forms the basis for a significant proportion of commercial systems. These transformers are generally what is referred to as “step down transformers” for converting high voltage—low current inputs to lower voltage—higher current outputs.
However, in a range of other applications within electrical systems and measurement systems what is required are precision AC current sources and AC amplifiers. The inventors have found that improvement of the accuracy when designing a precision AC current source is a different problem to measurement systems in that we either wish to remove measuring equipment connected to the output circuit to provide the feedback or wish that the generation and measurement of even very large current AC current sources is performed without requiring the use of a shunt.
Accordingly, the inventors have established design and circuit methodologies which are applicable to precision AC current sources, amplifiers, and also AC current measurements. Such measurements include precision AC current, voltage, phase, impedance, frequency, power and energy measurements, over current ranges from 1 mA or less to 20 kA or greater and voltage ranges of 1V or less to 1000 kV or greater and over frequency ranges from a few hertz to hundreds of kilohertz. Similarly, precision AC current sources and amplifiers for test, measurement, and supply applications are desirable over current ranges from 1 mA or less to 20 kA or greater and voltage ranges of 1V or less to 1000 kV or greater and over frequency ranges from a few hertz to hundreds of kilohertz.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.